Atmospheric Air Pollutant Dispersion

Atmospheric Air Pollutant
• Atmospheric Stability
Dispersion
Estimate air pollutant concentrations
downwind of emission point sources.
Downwind Air Pollution Concentrations
Are a function of:
• Air Temperature Lapse Rates
• Atmospheric Air Inversions
• Atmospheric Mixing Height
• Dispersion from Point Emission Sources
• Dispersion Coefficients
In CEE490/ENVH 461 class we will
Primarily use EPA SCREEN software

Atmospheric Air Vertical Stability
Dry Adiabatic
=
Lapse Rate
ΔTemp
ΔAltitude
ΔT
ΔZ
• Adiabatic vertical air movement causes a
change in pressure and temperature:
– dT/dz = g/C p = γ d (dry adiabatic lapse rate)
γ d = 9.8 K/km
--Stable lapse rate: γ < γ d
– Unstable lapse rate: γ > γ d
=
o
9.76K
=
=
1000meters
o
5.4F
1000ft

Atmospheric Stability
Characterized by vertical temperature
gradients (Lapse Rates)
– Dry adiabatic lapse rate (Γ) = 0.976 o C/100 m ~
1 o C/100 m
– International standard lapse rate = 0.0066 o C/m
Does the air temperature lapse rate have anything to
do with air quality?
Yes, because it is related to amount of vertical mixing
of emitted air pollutants.

• First Law of Thermodynamics
= 0 for adiabatic expansion
dq = dh −υdP
= C pdT
• Barometric Equation
dP
= −ρg
dZ
1
⇒ C pdT
= dP = −gdZ
ρ
⇒
dT
dZ
=
−
g
C
p
−
Lapse Rate
1
dP
ρ

100 m
Elevation
(m)
Air Temperature Lapse Rates
Superadiabatic
Superadiabatic
Dry Adiabatic Lapse Rate
Subadiabatic
Subadiabatic
T
Temperature ( oC) Inversion Inversion

Stability Conditions
Adiabatic lapse rate
Actual Air Temperature lapse rate

z
1 km
Mixing
depth
0
NIGHT
Diurnal Cycle of Surface
MIDDAY
Heating /Cooling
Subsidence
inversion
MORNING
T
NIGHT MORNING AFTERNOON

Actual Temperature Sounding

Superadiabatic Lapse Rates (Unstable air)
• Temperature decreases are greater than -10 o C/1000 meters
• Occur on sunny days
• Characterized by intense vertical mixing
• Excellent dispersion conditions

Atmospheric Stability
Superadiabatic – Strong Lapse Rate
Unstable Conditions

Neutral Air Temp Lapse Rates
• Temperature decrease with altitude is similar to the adiabatic lapse rate
• Results from:
– Cloudy conditions
– Elevated wind speeds
– Day/night transitions
• Describes OK dispersion conditions
Isothermal Lapse Rates (Weakly Stable)
• Characterized by no temperature change with height
• Atmosphere is somewhat stable
• Dispersion conditions are moderate

Atmospheric Stability
Subadiabatic – Weak Lapse Rate
Stable Conditions

• 2 major types of inversion:
Subsidence: descent of a layer of air within a high pressure
air mass (descending air increases pressure & temp.)
Radiation: thermal radiation at night from the earth’s
surface into the clear night sky
Radiation
Inversion

Inverted Air Temp Lapse Rates (Strongly Stable)
Characterized by increasing air temperature with height
Does it occur during the day or at night? (Both)
Is it associated with high or low air pressure systems? (High)
Does it improve or deteriorate air quality? (deteriorate)
www.ew.govt.nz/enviroinfo/air/weather.htm
Inversion
www.co.mendocino.ca.us/aqmd/Inversions.htm

• Inversion: Air Temperature increases with altitude

http://www.co.mendocino.ca.us/aqmd/pages/Inversion-Art-(web).jpg

Radiation Inversions
• Result from radiational cooling of the ground
• Occur on cloudless nights and clear sky – nocturnal
• Are intensified in valleys (heavier cooled air descends to valley floor)
• Cause air pollutants to be “trapped” (poor vertical transport)
www.co.mendocino.ca.us/aqmd
/Inversions.htm
What happens to inversion when sun rises?

Radiation Inversions
• Inversion Breaks up after sunrise
• Breakup results in elevated ground level concentrations
• Breakup described as a fumigation
Red Line is Air Temperature
Blue Line is adiabatic air temp lapse

Radiation Inversions
• Elevated inversions are formed over urban areas
– Due to heat island effect

Subsidence Inversion
• Associated with atmospheric high-pressure systems
• Inversion layer is formed aloft due to subsiding air
• Persists for days

Subsidence Inversion
• Migrating high-pressure systems: contribute to the hazy
summer conditions
• Semi-permanent marine high-pressure systems
– Results in a large
number of sunny calm
days
– Inversion layer closest to
the ground on
continental side
– Responsible for air
stagnation over
Southern California
www.oceansatlas.org/.../datard.htm

• Advective - warm air flows over a cold surface

• Mixing Height = Height of air that is mixed and where dispersion occurs
What is the Mixing Height in a radiational inversion?
When does the max MH occur during a day? Min MH?
Which season has the max MH? Min MH?
Why does Phoenix have a larger MH than New Orleans?
Why is agricultural burning allowed only during daytime?

Air Pollutant Dispersion from Point Sources
• Plume rise affects dispersion and transport
– Affects maximum ground level concentrations
– Affects distance to maximum ground level conc.

Lapse Rates and Atmospheric Stability
Weak Lapse Condition (Coning)
Wind
Z = altitude
T = Temp
Γ = adiabatic lapse rate

Stack Plume: Coning
Strong wind, no turbulence
What is the stability class? (dashed line is adiabatic
lapse rate) C
Is there good vertical mixing? OK
On sunny or cloudy days? Partly cloudy
Good for dispersing pollutants? OK

Lapse Rates and Atmospheric Stability
Inversion Condition (Fanning)
Wind

Stack Plume: Fanning
What is the stability class? (solid line is actual air
temperature with altitude air temp lapse rate)
What is the top view of the plume?
http://www.med.usf.edu/~npoor/4

Stack Plume: Fumigation
Why can’t the pollutants be dispersed upward?
Plume trapped by inversion above stack height.
Does it happen during the day or night? Morning

Lapse Rates and Atmospheric Stability
Inversion Below, Lapse Aloft (Lofting)
Wind

Stack Plume: Lofting
Why can’t the pollutants be dispersed downward?
What time of the day or night does this happen?
Evening – night as radiation inversion forms

Lapse Rates and Atmospheric Stability
Weak Lapse Below, Inversion Aloft (Trapping)
Wind

Stack Plume: Trapping
What weather conditions cause plume trapping?
Radiation inversion at ground level, subsidence
inversion at higher altitude (evening – night)

Stack Plume: Looping
Strong turbulence
http://www.med.usf.edu/~npoor/3
Is it at stable or unstable condition? Unstable
High or low wind speed? Low wind speed.
Does it happen during the day or night? Day
Is it good for dispersing pollutants? Yes

Dilution of Pollutants in the Atmosphere
• Air movement can dilute and remove
pollutants (removal by absorption and
deposition by snow, rain, & to surfaces)
• Pollutant dilution is variable, from quite
good to quite poor, according to the wind
velocity and the air stability (lapse rate).

Characteristics of Dispersion Models
• The accuracy of air pollutant dispersion models
varies according to the complexity of the
terrain and the availability of historic
meteorological data.
• The acceptability of the results of dispersion
models varies with the experience and
viewpoint of the modeler, the regulator and the
intervener.

Air Quality Modeling
Gaussian Dispersion Model
EPA Air Quality Models (SCREEN TSCREEN,ISC, AERMOD, etc.)
are computer software with equation parameters that include
pollutant emission rate Q (gms/sec), stack height (meters), stack
inside diameter at exit, stack gas temp, stack gas exit velocity
(m/s) ambient air temp, receptor height (m), topography, etc.
and calculate the downwind air pollutant concentrations.
The EPA dispersion software models are used to:
1. Evaluate compliance with NAAQS & prevention of
significant air quality deterioration (PSD required for permit
to construct)
2. Find pollutant emission reductions required.
3. Review permit to construct applications.

Is the Air Quality OK?
What is the level of my exposure to these emissions?
Is my family safe?
Where is a safe location with regards to air quality?
How about the adverse impact on the environment
(plants, animals, buildings)?
How to predict the impact of air pollutant emissions
resulting from population growth?
Where is the cleanest air; in city center, in rural area?
Note: Children health (respiratory effects) has been
correlated to the distance from their home to nearest
highway or busy street (diesel engine emissions)

When are model applications required for regulatory
purposes?
• SCREEN3, TSCREEN,
• ISC (Industrial Source Complex),
• AERMOD
AERMOD stands for American merican Meteorological Society
Environmental nvironmental Protection Agency Regulatory egulatory Model Model
Formally Proposed as replacement for ISC in 2000
Adopted as Preferred Model November 9, 2005

Regulatory Application of Models
• PSD: Prevention of Significant Deterioration of Air Quality
in relatively clean areas (e.g. National Parks, Wilderness
Areas, Indian Reservations)
• SIP: State Implementation Plan revisions for existing
sources and for New Source Reviews (NSR)

Classifications of Air Quality Models
• Developed for a number of air pollutant types and
time periods
– Short-term models – for a few hours to a few
days; worst case episode conditions
– Long-term models – to predict seasonal or
annual average concentrations; health effects
due to exposure
• Classified by
– Non-reactive models – pollutants such as SO 2 and CO
– Reactive models – pollutants such as O 3 , NO 2 , etc.

Air Quality Models
• Classified by coordinate system used
– Grid-based
• Region divided into an array of cells
• Used to determine compliance with NAAQS
– Trajectory
• Follow plume as it moves downwind
• Classified by sophistication level
– Screening: simple estimation use preset, worstcase
meteorological conditions to provide
conservative estimates.
– Refined: more detailed treatment of physical and
chemical atmospheric processes; require more
detailed and precise meteorological and
topographical input data.
http://www.epa.gov/scram001/i
mages/smokestacks.jpg

US EPA Air Quality Models
• Screening models available at:
www.epa.gov/scram001/tt22.htm#screen
• Preferred models available at:
http://www.epa.gov/scram001/tt22.htm#rec
– A single model found to outperform others
• Selected on the basis of other factors such as past use,
public familiarity, cost or resource requirements and
availability
• No further evaluation of a preferred model is required

Gaussian Dispersion Models
• Most widely used
• Based on the assumption
– plume spread results primarily by diffusion
– horizontal & vertical pollutant concentrations in
the plume have double Gaussian distribution)
Q
u
H
z
x
y

Gaussian Model Assumptions
• Gaussian dispersion modeling based on a number of
assumptions including
– Source pollutant emission rate = constant (Steady-state)
– Constant Wind speed, wind direction, and atmospheric
stability class
– Pollutant Mass transfer primarily due to bulk air
motion in the x-direction
– No pollutant chemical transformations occur
– Wind speeds are >1 m/sec.
– Limited to predicting concentrations > 50 m downwind

Gaussian Dispersion Model

Characteristics of Pollutant Plume
Horizontal (y) and vertical (z) dispersion, is
caused by eddies and random shifts of wind
direction.
• Key parameters are:
– Physical stack height (h) – Plume rise (Δh)
– Effective stack height (H) – Wind speed (ux )

Plume Dispersion Coordinate System

The Gaussian Model
• C = C(x, y, z, stability)
C
⎛ 2 ⎞
2
Q
⎧ ⎛ ⎞ ⎛
⎜
1 y
⎟
1 ( z − H ) 1 ( z + H )
exp − ⎨exp
⎜
⎜−
⎟ + exp ⎜
⎜−
⎜ 2 ⎟
2
2πσ yσ
z ⎝ 2 σ y ⎠⎩
⎝ 2 σ z ⎠ ⎝ 2 σ z
= 2
• σ y and σ z depend on the atmospheric conditions
• Atmospheric stability classifications are defined
in terms of surface wind speed, incoming solar
radiation and cloud cover
2
⎞⎫
⎟
⎟⎬
⎠⎭

C
Gaussian Dispersion Equation
( x,
y,
z)
⎡ ⎛ 2
Q 1
⎢ ⎜
y
= exp − +
πσ σ ⎢
⎜ 2
2
2 y zu
⎣
2 ⎝ σ y σ z
( z − H )
σ y & σ z = f(downwind distance x & atmos stability)
• Q = pollutant emission rate (grams/sec)
• H = effective stack height (meters) = stack height + plume rise
• u = wind speed (m/sec)
σ y = horizontal crosswind dispersion coefficient (meters)
σ z= vertical dispersion coefficient (meters)
2
⎞⎤
⎟
⎥
⎠⎥⎦

Plume Dispersion Equations
General Equation – Plume with Reflection for Plume Height H
2
( z H ) ⎞ ⎛ ( z + H )
2
Q ⎡ ⎛ y ⎞ ⎧ ⎛
C(
x,
y,
z;
H ) = • ⎢exp⎜
⎟
⎜
− • ⎨ ⎜
⎟
exp −
2
2πuσ σ ⎢⎣
⎝ 2σ
y z
y ⎠ ⎩ ⎝
−
2
2σ
z
⎟ + exp ⎜
⎜−
⎠ ⎝
2
2σ
z
Ground Level Concentration – Plume at Height H
C(
x,
y,
0;
H )
2
2
Q ⎡ ⎛ ⎞ ⎛ ⎞⎤
⎢ ⎜
y H
• exp ⎟
⎜
−
⎟
• exp⎜
− ⎟⎥
πuσ
σ ⎣⎢
⎝ 2σ
⎠ ⎝ 2σ
y z
y
z ⎠⎥⎦
= 2
2
Ground Level Center Line Conc (y = 0) – Plume Height H
C(
x,
0,
0;
H )
2
Q ⎡ ⎛ H ⎞⎤
• ⎢exp⎜
− ⎟⎥
πuσ
σ ⎣ ⎝ 2σ
y z
z ⎠⎦
= 2
Ground Level Center Line – Ground Point Source (y = 0, H = 0)
Q
C(
x,
0,
0;
0)
=
πuσ
σ
y
z
2
⎞⎫⎤
⎟
⎟⎬⎥
⎠⎭⎥⎦

Gaussian Dispersion Equation
If the emission source is at ground level with no
effective plume rise then
C
( x,
y,
z)
⎡ ⎛ 2 2
Q 1
⎢ ⎜
y z
= exp − +
πσ σ ⎢
⎜ 2 2
y zu
⎣
2 ⎝ σ y σ z
Plume Rise
• H is the sum of the physical stack height and plume rise.
H
=
Δh
plume rise
+
h
actual
stack
⎞⎤
⎟
⎥
⎠⎥⎦

Key to Stability Categories

Atmospheric Stability Classes

Horizontal Dispersion Coefficient σ y

Vertical Dispersion Coefficient σ z

Maximum Downwind Ground-Level
Concentration (C max )
• Pollutants require time (and distance) to reach the
ground.
• C max decreases as effective plume height H
increases.
• Distance to C max increases as H increases.

Maximum Concentration (C max ) and Distance to
C max (x max )

Maximum Ground Level Concentration
Under moderately stable to near neutral conditions,
σ y = 1
k σ
z
The ground level concentration at the center line is
2
Q ⎡ H ⎤
C x,
0,
0 = exp⎢−
2
2
πk1σ
z u ⎣ 2σ
z
( ) ⎥ ⎦
The maximum occurs at
dC / dσz
= 0 ⇒ σz
=
H
2
Once σ z is determined, x can be known and subsequently C.
C
Q
= = 0.
1171
πσ σ u
σ
( x,
0,
0)
exp[
−1]
y
z
Q
σ
y
z
u

Plume Rise
• H is the sum of the physical stack height and plume rise.
H
=
Δh
plumerise
+
h
actualstack

Pollutant Plume Rise
Plume rises from stack exit.

Plume Rise
Buoyant plume: Initial buoyancy >> initial momentum
Forced plume: Initial buoyancy ~ initial momentum
Jet: Initial buoyancy 55 m / s )
where buoyancy flux is
2
F = gV d ( T −T
) / 4T
s
u
a
S
)
V s: Stack exit velocity, m/s
d: top inside stack diameter, m
T s: stack gas temperature, K
T a: ambient temperature, K
g: gravity, 9.8 m/s 2

Carson and Moses: vertical momentum & thermal
buoyancy, based on 615 observations involving 26 stacks.
Δh
Δh
Δh
plume rise
plume rise
plume rise
=
=
3.
47
0.
35
Vsd
u
Vsd
u
Vsd
= −1.
04
u
( T T )
Q = m&
C −
h
m&
=
πd
4
2
p
s
P
Vs
RT
s
a
+
+
5.
15
2.
64
+
2.
24
Q
u
Q
u
h
h
Q
u
h
(unstable)
(neutral)
(stable)
(heat emission rate, kJ/s)
(stack gas mass flow rate. kg/s)

C
( x,
y,
z)
Q
2πσ
σ
⎡ y
exp⎢−
⎢⎣
2σ
2
Wark & Warner, “Air Pollution: Its Origin & Control”
⎤⎪⎧
⎡
⎥⎨exp⎢−
⎥⎦
⎪⎩ ⎣
2 ( z − H ) ⎤ ⎡ ( z + H )
= 2
2
2
y zu
y 2σz
2σz
⎥ + exp⎢−
⎦ ⎣
Plume is Reflected when it touches ground surface.
2
⎤⎪⎫
⎥⎬
⎦⎪⎭

Ground level concentration
C
Q
πσ σ
⎡
exp⎢−
⎢⎣
y
2σ
⎤ ⎡
⎥ exp⎢−
⎥⎦
⎣
H
2σ
= 2
2
y zu
y
z
2
2
⎤
⎥
⎦

Dispersion of SO 2 from Stack
• An industrial boiler is burning at 12 tons of 2.5% sulfur coal/hr
with an emission rate of 151 g/s. The following exist : H = 120
m, u = 2 m/s, y = 0. It is one hour before sunrise, and the sky
is clear. Find the downwind ground level SO2 concentration at
x =2 km, y = 0, and z = 0.
Stability class =
σy =
σz =
C(x=2 km, y=0, z=0) =
C
Q
πσ σ
⎡ 2
y
exp⎢−
⎢⎣
2σ
⎤ ⎡ H
⎥exp⎢−
⎥⎦
⎣ 2σ
= 2
2
y zu
y
z
2
⎤
⎥
⎦

Dispersion of Ground Level Air Pollutant Emissions
• Air pollutant emissions are from a ground level source
with H = 0, u = 4 m/s, Q = 100 g/s, and the stability class
= B, what is downwind concentration at x = 200 m, y = 0,
and z = 0?
At 200 m:
σ y =
σ z =
C(x=200 m, y=0, z=0) =

• Calculate H using plume rise equations for an 80 m high stack (h) with a stack
diameter = 4 m, stack gas velocity = 14 m/s, stack gas temperature = 90o C (363
K), ambient temperature = 25 oC (298 K), 10 meter high wind speed u at 10 m
= 4m/s, and stability class = B. Find the Max Ground Level air pollutant
concentration & its (location downwind distance x).
F =
Δh plume rise =
H =
σz =
σy =
Cmax =
Plume Rise and Max Conc
Residents around the Rock Cement Plant are complaining that its emission
are in violation and the ground level concentrations exceed the air quality
standards. The plant has its facility within 0.2 km diameter fence. Its
effective stack height is 50 m. You are a government agency environmental
engineer. Where are you going to locate your air quality monitors? Why?

Complex Horizontal, Vertical, and Temporal Wind Structure
Winds aloft have no continuous measurements

Meteorology

In most of cases terrain is not flat terrain – topographic complexity
Complex horizontal, vertical, and temporal dispersion

Plume Air Pollutant Building Downwash

Building Downwash

Building Downwash

Building Downwash – Short Stack

Building Downwash – Taller Stack (Not GEP)

Building Downwash – Tallest Stack
H GEP = 2.5 H Bldg Prior to 1979 H GEP = H Bldg + 1.5 L

Cavity and Wakes

Cavity and Far Wake
Cavity

Building Downwash for 2 Identical Stack Emissions at Different Locations
Building
Cavity
The stack on the left is located on top of a building and this structure affects the
wind-flow which, in turn, affects the plume trajectory, pulling it down into the
cavity zone (near wake) behind the building. The stack on the right is located far
enough downwind of the building to be unaffected by the cavity (near wake)
effects and is only affected by the air flow in the far wake.

Building Downwash
Cavity

Z R
Cavity
X R
2Y R
Cavity and Wakes
Z R
2Y R

Aerodynamic Wake
• Region where local air velocities are different from
the free stream values
• Streamline Separation – at an object
– Eddy Recirculation (generally lower velocity region)
– Turbulent Shear Region (generally higher velocity)
• Reattachment of Streamlines
• Near Wake (Cavity)
– Usually on the ‘lee’ side of the object.
• Far Wake
– Effect of another object on the separated streamlines
• Estimation of Wake/Cavity Boundary
– Effect of Building geometry

Two Points of Concern
• Dispersion of Air Pollutant Plume in the presence
of Buildings
– What happens to the plume from an existing stack and
existing buildings?
• Design of Stacks in the Presence of Building
– What are the design guidelines if a new emission source
is being proposed in an area full of buildings?

Good Engineering Practice GEP Analysis
• H GEP = good engineering practice height
• H GEP = H + 1.5 L
– H = height of adjacent or nearby structure
– L = lesser dimension height or proj. width
– 5L = region of influence
• The structure with the greatest influence
is then used in the model to evaluate
wake effects and downwash.

Good Engineering Practice Stack Height
1985 Regulations -
GEP Stack height – greater of the following
- a) 65 m from the base of the stack
-b) H GEP = 2.5 H
for stacks in existence before Jan, 1979.
For All Other Stacks:
H GEP = H + 1.5 L
Where H is the height of the nearest building and
L = lesser dimension height or projected building width

GEP stack height
• Building downwash can occur when
H Stack = H S < H b + 1.5L
H Stack = Height of Stack
H b = Height of Building
L = lesser of H b or PBW
PBW = Maximum Projected Building Width
Screen models will do this calculation when the
building downwash option is used. If HS > Hb +
1.5L, then building downwash will not be shown in
SCREEN results

Getting Started – Screen 3
• Convert all lengths and distances to meters
• Convert temperatures to degrees Kelvin
• Identify building contributions to air
dispersion (stack emissions)
• Screen 3 should be run in regulatory
default mode
• Note that SCREEN3 differs from TSCREEN (SCREEN3
can select atmospheric stability classes or run Full
Meteorology, will not calculate concentrations at other
averaging times, etc.)

Information Required to run
Screen Models for Point Source
• Emission rate (g/s)
• Stack Height (m)
• Shortest distance to property line
• Stack velocity (or volumetric airflow SCREEN3)
• Stack gas temperature ( o K)
• Stack Inside Diameter
• Building Height, Length, Width

SCREEN Example – Stack emissions
• Emission rate (g/s) .01 g/s
• Stack Height 15.24m
• Building Height 6.096m
• Shortest Distance to property line 91.44m
• Stack airflow in acfm 20,000 acfm
• Stack gas temperature 294.3° K
• Stack inside diameter 1.143m
• Building dimensions 30.48m L, 30.48m W, 10.67m H

SCREEN Example – Stack emissions
Note that this is the output from SCREEN3 software (not TSCREEN)

SCREEN Example – Stack emissions

SCREEN Example – Stack emissions

SCREEN Example – Stack emissions

SCREEN Example – Stack emissions

SCREEN Example – Stack emissions

SCREEN Example – Stack emissions
The most conservative scenario gives a maximum 1-hr 1 hr
concentration of 2.595 ug/m 3 at a distance of 91 meters

Flare Emissions Elevated point source

Screen Model for Flare Source
• Emission Rate
• Flare Stack Height
• Total Heat Release Rate
• Shortest Distance to property line
• Influential Building Dimensions

SCREEN Example – Flare emissions

SCREEN Example – Flare emissions

SCREEN Example – Flare emissions

SCREEN Example – Flare emissions

Screen Model for Area Source
• Emission Rate
• Source Release Height
• Larger Side Length of Rectangular Area
• Smaller Side Length of Rectangular Area
• Shortest Distance to property line

SCREEN Example – Area Source

SCREEN Example – Flare emissions

Screen Model for Volume Source
• Emission Rate
• Source Release Height
• Initial Lateral Dimension
• Initial Vertical Dimension
• Shortest Distance to Property Line

Volume Source
• Source Release Height is the center of the
Volume Source:
If the Source is from a building, the release
height is set equal to one half of the building
height.
• Volume sources are modeled as a square in
Screen3. If the source is not square, the width
should be set to the minimum length.

Volume Source
Initial Lateral Dimension (σ y0 )
Single Volume Source
σ y0 = length of side divided by 4.3
Line Source composed of several volume sources
σ y0 = length of side divided by 2.15
Line Source composed of separated volume sources
σ y0 = center to center distance divided by 2.15

Volume Source
Initial Vertical Dimension (σ z0 )
Surface-Based Source
σ z0 = vertical dimension of source divided by 2.15
Elevated Source on or adjacent to a building
σ z0 = building height divided by 2.15
Elevated Source not on or adjacent to a building
σ z0 = vertical dimension of source divided by 4.3

Vertical Dimension is
height of building
divided by 2.15
Example – Volume Source
Volume Source from a Building
Lateral Dimension is
minimum length of
building divided by 4.3
Release Height is ½ of
Building Height

SCREEN Example – Volume Source

SCREEN Example – Volume Source
Initial Lateral Dimension obtained by taking
building length of 30.48m divided by 4.3
Using a building as the volume source, so the
initial vertical dimension is the height of the
building divided by 2.15

SCREEN Example – Volume Source

SCREEN Example – Volume Source

Screen model results
• Screen model results are all
maximum 1-hr concentrations (except
for complex terrain and if SCREEN is
run inside of TSCREEN can obtain
concentrations in other averaging
times).